Adaptive engine control for low emission vehicle starting

ABSTRACT

A method for rapidly heating an emission control device in an engine exhaust uses excess air added to the exhaust via an air introduction device. After an engine cold start, the engine is operated to raise exhaust manifold temperature to a first predetermined value by operating the engine with a lean air-fuel ratio and retarded ignition timing. Once the exhaust manifold reaches the predetermined temperature value, the engine is switched to operate rich and air is added via the air introduction device. The added air and rich exhaust gas burn in the exhaust, thereby generating heat and raising catalyst temperature even more rapidly. The rich operation and excess air are continued until either engine airflow increases beyond a pre-selected value, or the emission control device reaches a desired temperature value. After the emission control device reaches the desired temperature, the engine is operated substantially around stoichiometry. Further, a method is described for adaptively learning pump airflow using feedback from an exhaust gas oxygen sensor.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to co-pending application Ser. No.______, Docket number 202-0582, titled “ENGINE CONTROL FOR LOW EMISSIONVEHICLE STARTING”, filed on the same day as this application, and havinga common assignee with this application, the entire contents of whichare hereby expressly incorporated by reference; and co-pendingapplication Ser. No. ______, Docket number 202-0754, titled “METHOD FORLOW EMISION VEHICLE STARTING WITH IMPROVED FUEL ECONOMY”, filed on thesame day as this application, and having a common assignee with thisapplication, the entire contents of which are hereby expresslyincorporated by reference.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the control of aninternal combustion engine having an air introduction device in theengine exhaust.

[0004] 2. Background of the Invention

[0005] Vehicle engines have used a secondary air pump in the engineexhaust to introduce air to increase emission control. The pumps areattached, for example, to the exhaust manifold of the engine and pumpambient air into the exhaust. The secondary air can reach with richexhaust gasses to rapidly heat the catalytic converter. Further, schemeshave been developed for estimating the airflow introduced via thesecondary air pumps. An example of such a system is described in U.S.Pat. No. 6,044,643.

[0006] The inventors herein have recognized a disadvantage with theabove approach. In particular, as vehicle conditions change andcomponents age, accuracy of open loop estimates can vary. Degradation inthe estimate of airflow introduced via a secondary air pump can haveresult in degraded emission control. This is especially true in a systemthat attempts to exothermically react rich exhaust gasses with secondaryair. In other words, if the estimate of airflow is inaccurate, theoverall mixture may provide incomplete exothermic reaction, resulting indegraded emissions.

SUMMARY OF INVENTION

[0007] The above disadvantages are overcome by a method for controllingengine operation, the engine having an exhaust system including an airintroduction device and an exhaust gas sensor. The method comprises:after an engine cold start, operating the engine with a rich air-fuelratio and adding air via said air introduction device; and during saidoperation: estimating an air amount introduced into the exhaust fromsaid air introduction device based at least on an operating conditionand an adaptively learned parameter; determining if the sensor isoperating; in response to said determination, updating said adaptivelylearned parameter based on said exhaust gas sensor; and injecting a fuelinjection amount into the engine based on said estimated air amount.

[0008] By using an adaptive approach, it is possible to correct forvariations in pump flow due to pump aging, changing conditions inparameters that are not included in an open loop estimate, and modelingerrors. Further, by adapting the model during certain conditions, it ispossible to compensate for errors caused by other factors, such asoffsets in the fuel injectors at a time when pump flow estimation errorsdo not exist (since the pump is not operating). Then, when the pump isoperating and adaptation is enabled, the present invention can assignany error to estimations in pump flow. In this way, a robust controlsystem can be obtained.

[0009] Note that various types of adapative parameters can be used. Forexample, a single parameter can be used that accounts for all modelingerror. Alternatively, adaptive data across various engine orenvironmental operating conditions can be used.

BRIEF DESCRIPTION OF DRAWINGS

[0010] The advantages described herein will be more fully understood byreading an example of an embodiment in which the invention is used toadvantage, referred to herein as the Description of the PreferredEmbodiment, with reference to the drawings wherein:

[0011]FIG. 1 is a block diagram of an engine in which the invention isused to advantage;

[0012] FIGS. 2-8 and 13-14 are high level flowcharts of various routinesfor controlling engine and components according to the presentinvention;

[0013] FIGS. 9-11 and 15 are graphs illustrating operation according toone aspect of the present invention; and

[0014]FIG. 12 shows details of the air introduction device.

DETAILED DESCRIPTION

[0015] Internal combustion engine 10, comprising a plurality ofcylinders, one cylinder of which is shown in FIG. 1, is controlled byelectronic engine controller 12. Engine 10 includes combustion chamber30 and cylinder walls 32 with piston 36 positioned therein and connectedto crankshaft 40. Combustion chamber 30 communicates with intakemanifold 44 and exhaust manifold 48 via respective intake valve 52 andexhaust valve 54. Exhaust gas oxygen sensor 16 is coupled to exhaustmanifold 48 of engine 10 upstream of catalytic converter 20. In generalterms, which are described later herein, controller 12 controls engineair/fuel ratio in response to feedback variable FV derived fromtwo-state exhaust gas oxygen sensor 16.

[0016] As described above, two-state exhaust gas oxygen sensor 16 isshown coupled to exhaust manifold 48 upstream of catalytic converter 20.Two-state exhaust gas oxygen sensor 80 is also shown coupled to exhaustmanifold 48 downstream of catalytic converter 20. Sensor 16 providessignal EGO1 to controller 12 which converts signal EGO1 into two-statesignal EGOS1. A high voltage state of signal EGOS1 indicates exhaustgases are rich of a reference air/fuel ratio and a low voltage state ofconverted signal EGO1 indicates exhaust gases are lean of the referenceair/fuel ratio. Sensor 80 provides signal EGO2 to controller 12 whichconverts signal EGO2 into two-state signal EGOS2. A high voltage stateof signal EGOS2 indicates exhaust gases are rich of a reference air/fuelratio and a low voltage state of converted signal EGO1 indicates exhaustgases are lean of the reference air/fuel ratio.

[0017] Intake manifold 44 communicates with throttle body 64 viathrottle plate 66. In one embodiment, an electronically controlledthrottle can be used without an air bypass valve. In this case, theairflow is controlled via the throttle instead of using the idle airbypass valve around the throttle plate. If a mechanical throttle (linkedvia a wire to pedal 70) is used, then the air bypass valve is used toelectronically adjust air around the throttle 66, as is known in theart. Intake manifold 44 is also shown having fuel injector 68 coupledthereto for delivering fuel in proportion to the pulse width of signal(fpw) from controller 12. Fuel is delivered to fuel injector 68 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown). Engine 10 further includes conventionaldistributorless ignition system 88 to provide ignition spark tocombustion chamber 30 via spark plug 92 in response to controller 12. Inthe embodiment described herein, controller 12 is a conventionalmicrocomputer including: microprocessor unit 102, input/output ports104, electronic memory chip 106, which is an electronically programmablememory in this particular example, random access memory 108, and aconventional data bus.

[0018] Controller 12 receives various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:measurements of inducted mass air flow (MAF) from mass air flow sensor110 coupled to throttle body 64; engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling jacket 114; a measurement ofmanifold pressure (MAP) from manifold pressure sensor 116 coupled tointake manifold 44; a measurement of throttle position (TP) fromthrottle position sensor 117 coupled to throttle plate 66; and a profileignition pickup signal (PIP) from Hall effect sensor 118 coupled tocrankshaft 40 indicating and engine speed (N). Further, controller 12receives a measurement of manifold temperature (Te) from sensor 76.Alternatively, sensor 76 can provide an indication of exhaust gastemperature, or catalyst temperature.

[0019] Further still, downstream sensor 80, provides a measurement ofair-fuel ratio downstream of emission control device 20. In one example,as described above, sensor 80 is a switching EGO type sensor. Inanother, sensor 80 provides an indication of air-fuel ratio (or relativeair-fuel ratio) over a range of air-fuel ratios. In this case, thesensor 80 is known as a UEGO sensor.

[0020] Exhaust gas is delivered to intake manifold 44 by a conventionalEGR tube 202 communicating with exhaust manifold 48, EGR valve assembly200, and EGR orifice 205. Alternatively, tube 202 could be an internallyrouted passage in the engine that communicates between exhaust manifold48 and intake manifold 44. Pressure sensor 206 communicates with EGRtube 202 between valve assembly 200 and orifice 205. Pressure sensor 207communicates with intake manifold 44. Stated another way, exhaust gastravels from exhaust manifold 44 first through valve assembly 200, thenthrough EGR orifice 205, to intake manifold 44. EGR valve assembly 200can then be said to be located upstream of orifice 205. Also, pressuresensor 206 can be either absolute pressure sensor or a gauge pressuresensor. Further, pressure sensor 207 can be either an absolute pressuresensor or a gauge pressure sensor. Further yet, pressure sensor 206 canbe an absolute pressure sensor, while pressure sensor 207 can be a gaugepressure sensor.

[0021] The flow sensors provide a measurement of manifold pressure (MAP)and pressure drop across orifice 205 (DP) to controller 12. Signals MAPand DP are then used to calculate EGR flow. EGR valve assembly 200 has avalve position (not shown) for controlling a variable area restrictionin EGR tube 202, which thereby controls EGR flow. EGR valve assembly 200can either minimally restrict EGR flow through tube 202 or completelyrestrict EGR flow through tube 202. Vacuum regulator 224 is coupled toEGR valve assembly 200. Vacuum regulator 224 receives actuation signal(226) from controller 12 for controlling valve position of EGR valveassembly 200. In a preferred embodiment, EGR valve assembly 200 is avacuum actuated valve. However, any type of flow control valve may beused such as, for example, an electrical solenoid powered valve or astepper motor powered valve.

[0022] Finally, air introduction device 209 is coupled to exhaustmanifold 48, upstream of catalyst 20. Controller 12 sends a commandvoltage (Vp) to control device 209. Device 209 can be an air pump, whichpumps ambient air into exhaust manifold 48. The amount of air pumpeddepends on the command signal voltage (Vp). Signal Vp can be a voltagesignal, a duty cycle, a frequency modulated signal, or any other suchtype signal to transmit the command information to device 209. In oneexample, the air pump is an electrically powered device. More details ofthe exemplary system are described in more detail with reference to FIG.12.

[0023] Further, drive pedal 70 is shown, along with a driver's foot 72.Pedal position (pp) sensor 74 measures angular position of the driveractuated pedal.

[0024] Referring now to FIG. 2, a summary high-level flow chart isdescribed. This summary flow chart describes the vehicle/engine startingsequence using port oxidation (air introduced into the engine exhaust).The routine starts at step 210. Then, at step 212, the starting sequenceis initialized (see FIG. 3). Generally, environmental sensors such asengine coolant temperature, ambient air temperature, barometricpressure, battery voltage, and engine off time are used to calculatecompensation for the air pump (i.e., to compensate for temperature,pressure, and voltage). In other words, the present invention controlsvoltage Vp to obtain a desired amount of air introduced via pump 209.Since the amount of air for a given voltage depends on theseenvironmental conditions, the present invention adjusts the pump commandto compensate for variation in these environmental conditions. Forexample, at altitude, less air is introduced for a given voltage commandcompared to sea level. As such, when operating at altitude, a highervoltage is commanded to provide a given desired air introduction amount.Thus, the present invention achieves more accurate air estimate, aircontrol, and therefore more accurate open loop air-fuel control duringthe port oxidation mode.

[0025] Alternatively, this variation can be compensated for by adjustingfuel injection, with fuel injection also potentially being adjusted viafeedback from exhaust air-fuel ratio sensor. Such an approach isdescribed in FIGS. 13-15.

[0026] Further, these environmental sensors are also used to calculatefuel delivery and idle air bypass valve position (or position of anelectronic throttle, if equipped). Further, these parameters are used todetermine whether port oxidation is required.

[0027] Then, at step 214, the routine determines whether the engine iscranking. When the answer to step 214 is “no”, the routine returns tostep 212. Otherwise, when the engine is being cranked, the routinecontinues to step 216.

[0028] Step 216 describes the engine crank mode, which is more fullydescribed with regard to FIG. 4. In general terms, the crank mode isactive while the engine starter is engaged and the engine has notreached a point where it can maintain its speed. During this mode, thedesired air entering the engine, the fuel delivery, and ignition timingare scheduled in an open loop mode until the engine reaches apre-determined engine speed and combustion has stabilized to produceminimal hydrocarbons. This determination, of whether the engine is outof the crank mode, is made in step 218 based on engine speed reaching apreselected speed, for example. When the answer to step 218 is “no”, theroutine returns to step 216. Otherwise, the routine continues to step220.

[0029] In step 220, the routine determines whether the engine is cold.In other words, the routine determines whether the engine coolanttemperature is below a pre-determined temperature. When the answer tostep 220 is “yes”, the routine continues to the cold run mode in step222, which is described in detail with regard to FIG. 5. In general,during the cold run mode fuel is scheduled to produce a lean exhaustair-fuel mixture. Further, ignition timing is scheduled to a retarded(from optimal torque) value to minimize hydrocarbon emissions with acold catalyst. Since the retarded ignition timing and lean air-fuelcombustion produces less torque than advanced ignition timing and richair-fuel mixtures, engine speed may fall. To counteract this, engine rpmcan be maintained by increasing engine air flow, thereby increasingoverall cylinder air charge. Further, this increase in air flow has asmall additional benefit of providing a small additional amount of heatto the catalyst.

[0030] Next, in step 224, the routine determines whether port oxidationis required. Generally, this decision is based on information from anestimated exhaust flange temperature, catalyst temperature, diagnosticsensors, and the throttle (or pedal) state. If the catalyst and exhaustflange have reached a predetermined temperature, or a throttletransition has occurred before the auto ignition temperature is reached,port oxidation is disabled. If port oxidation is disabled, the enginewill continue to operate in the cold run mode until a determination ismade to enter the run mode. If port oxidation is enabled, the enginewill continue in cold run mode until the estimated exhaust flangetemperature is greater than the auto ignition temperature, for example,at which point the oxidation mode will be entered. This is describedmore fully with regard to FIG. 6.

[0031] When the answer to step 224 is “yes”, the routine continues tostep 226 to sequence the port oxidation, as described more fully withregard to FIG. 7. Generally, the routine considers various environmentalinputs to determine if port oxidation is required. If port oxidation isnot required, the cold run mode is maintained until the routinedetermines that run mode is appropriate. Cold run mode is calibrated sothat hydrocarbon emissions are minimized without drivability concerns.If port oxidation is required, a transition occurs from the cold runmode to the port oxidation mode. In this case, engine ignition isretarded further, and the injected fuel is increased to significantlyenrich the exhaust air-fuel ratio (as rich as 25% additional fuel).Further, the external air pump is enabled. Port oxidation is enableduntil a throttle input is detected, a target exhaust flange temperatureis reached, or a timer reaches a predetermined limit. When the autoignition temperature is reached, a second timer is started. At thispoint, fuel, air, and ignition timing can be individually controlled toachieve optimal hydrocarbon emissions. In one example, the enginediscontinues port oxidation by transitioning to stoichiometric operation(oscillations about stoichiometry via feedback air-fuel control usingoxygen sensors in the exhaust), i.e., maintaining the exhaust air-fuelratio about the stoichiometric point.

[0032] When transitioning to the port oxidation mode, the air-fuel ratiois switched to rich and the air pump is activated. However, theseactions may not occur at exactly the same time. In other words, sinceair pump hose lengths and pump capacities differ based on the particularconfiguration, one may require fuel to be initiated before air whileother configurations may require air before fuel.

[0033] Air pump flow is estimated by reading pump voltage, pressureacross the pump, and ambient pressure and temperature. I.e., air pumpcompensation for barometric pressure and external temperature can beachieved. The amount of fuel enrichment during port oxidation isdetermined from the estimated air pump air mass, with potentialadditional feedback using the oxygen sensor. In other words, during portoxidation, a desired engine air-fuel ratio is selected to achieveauto-ignition in the exhaust with the introduced air from the pump.Typically, this ratio is richer than about 12:1. Then, based on theactual combustion rich air-fuel ratio, and the air entering the engine(measured via sensor 110), a required air pump air flow is calculated.As an example, the following equation can be used to calculated thedesired air flow through the air pump:

air_pump_desired_air=(1λs/λc)*MAF

[0034] where λs is the stoichiometric air-fuel ratio and λc is the richcombustion air-fuel ratio. This provides a near stoichiometric mixtureof rich exhaust gasses with introduced air. Alternatively, a slightlylean or rich overall mixture can be used if desired. Further, thisdesired airflow is then used to determine a pump command voltage (Vp)based on ambient operating conditions as described above herein.

[0035] Additionally, the estimated exhaust gas temperature includescompensation for the heat generated by the exothermic reaction. While inthe port oxidation mode, the routine observes the throttle sensor todetermine if a transition has occurred. If a transition has occurred,the timer is stopped and fuel injection and ignition timing aretransitioned to the run mode conditions.

[0036] The inventors of the present invention have recognized thatdetecting transitions can provide improved operation. In particular, asthe driver moves the throttle, exhaust pressure raises thereby limitingair added by the air pump. In addition, the engine air mass increasessignificantly to the point where the external pump may not be able tosupply sufficient exhaust air, as described herein. Since the air pumpmay not be able to overcome the additional exhaust pressure, theexothermic reaction can be extinguished thereby increasing emissions.Therefore, port oxidation is disabled if a throttle transition occurs,the target temperature is reached, or a timer exceeds a predeterminedlimit. The shut-off sequence allows independent control of ignitiontiming, air, and fuel like the initiating sequence.

[0037] Next, in step 228 the routine transitions to the run mode asdescribed in more detail below with regard to FIG. 8. In general terms,since port oxidation and the cold run mode are substantially differentthan engine operation during the run mode, the engine is transitionedbetween the different modes. Exiting the cold run mode and the portoxidation mode requires moving from the retarded ignition timing to abase ignition timing. Therefore, the ignition timing is ramped over timeto minimize disturbing the engine speed/torque, which is beingmaintained during the idle conditions via adjustments to the idle airbypass valve. Further, port oxidation requires a rich air-fuel mixtureso that fuel is ramped toward stoichiometry to enter the run mode.Further, cold run mode utilizes a lean mixture to minimize hydrocarbons.Therefore, in this case, fuel is ramped rich to stoichiometry to enterthe run mode.

[0038] Engine air is reduced during the port oxidation due to theadditional torque provided by the rich air-fuel ratio. When portoxidation is exited, the ignition timing transition is done over asufficiently long time period so that the air flow reduction via theidle air bypass valve compensates to maintain the desired engine speed.Cold run mode requires excess engine air flow to maintain engine speed.As the engine warms, engine air is decreased as engine frictiondecreases thereby allowing the engine to maintain relatively constantspeed. To transition from the cold run mode to the run mode, bothignition timing and air from the idle air bypass are coordinated tomaintain substantially constant engine speed.

[0039] Finally, the routine continues to step 230 where the engineenters the run mode.

[0040] Referring now specifically to FIG. 3, the initialization of thestarting sequence of step 212 is described. First, in step 310, theroutine reads environmental conditions. For example, the routine readsthe engine coolant temperature, ambient air temperature, barometricpressure, battery voltage, and engine off timers. For example, theengine-off timers estimate the time since the last engine runningcondition. Then, in step 312, the routine calculates startingparameters. These starting parameters include the required fuelinjection amount and the initial idle air bypass valve position.Further, the starting parameters include compensation that may later beapplied to the air pump to account for pump temperature, pressure, andvoltage, as well as the ambient conditions. In other words, based on theinitial pump temperature, adjustment may be later made to the controlvalues sent to control the pump. As an example, if the initial pumptemperature is very cold due to cold environmental conditions,additional voltage may be needed to reach the same additional air flowthat could be provided at a lower voltage if the pump temperature werehigher.

[0041] Thus, the routine determines a required air amount to be providedby the pump, once it is commenced. To obtain the desired air quantityfrom the pump (which is determined from the rich air-fuel ratio that theengine will combust, as well as the engine air flow amount),compensation for environmental conditions is used. In an alternativeembodiment, the air pump operates at full airflow whenever activated andthe engine controller maintains exhaust air-fuel ratio by adjusting fuelinjection amount, or desired air-fuel ratio. I.e., the pump is simplyturned on and off, and the mixture ratio is maintained by adjusting thefuel injection amount. Continuing with FIG. 3, in step 314, the routineinitializes and activates outputs.

[0042] Referring now specifically to FIG. 4, the engine crank mode ofstep 216 is described. The routine first determines at step 410 whetherthe engine is turning. If not, the routine loops back to continuechecking whether the engine is turning. When the answer to step 410 is“yes”, the routine continues to step 412. In step 412, the routinesynchronizes fuel injection with the engine cylinder events. Next, instep 414, the routine outputs and determines the desired open loopignition timing, fuel injection, and idle air bypass commands. Inparticular, these initial open loop values can be determined based onparameters such as engine coolant temperature and intake air flow.Further, these parameters can be adjusted and calibrated to produceminimal hydrocarbons. Next, in step 416, the routine determines whetherthe engine has run up. In particular, the routine determines whether theengine has reached a predetermined engine speed threshold or whetherengine combustion has reached a predetermined level of stability. Whenthe answer to step 416 is “no”, the routine returns to step 414.Otherwise, when the answer to step 416 is “yes”, the routine ends.

[0043] Referring now specifically to FIG. 5, the cold run mode of step222 is now described. First, in step 510, the routine schedules a leanair-fuel mixture to be combusted in the engine. Typically the leanair-fuel ratio is only slightly lean. For example, typical values arefrom about 14.8-15:1 air-fuel ratios, where about 14.6 is nearlystoichiometry. Next, in step 512, the desired/scheduled engine air flowis increased. In other words, the idle air bypass valve (or electronicthrottle if equipped) is increased to provide additional air flow tocompensate for the lean combustion mixture and retarded ignition timingto thereby maintain engine speed. Next, in step 514, the retardedignition timing is scheduled.

[0044]FIG. 6 describes the determination of whether port oxidation isrequired in step 224. In particular, the routine of FIG. 6 determineswhether to enable additional air to be provided by the air pump in theengine exhaust. First, in step 610, the routine determines whether theestimated exhaust manifold flange temperature (ext_fl) is within apredetermined temperature range. In particular, the routine determineswhether the flange temperature is between a port oxidation lowtemperature and a port oxidation high temperature (Ox_low_tmp,Ox_hi_tmp). Note that alternative temperature indications can be used.For example, whether exhaust temperature or catalyst temperature isgreater than a pre-selected value for a predetermined time duration, ora predetermined number of engine events.

[0045] In an alternative embodiment, exhaust port temperature is used totrigger when to enable the air introduction device. Also, The portoxidation high temperature could be a light-off temperature of thecatalyst 20 above which no port oxidation nor cold run mode isnecessary.

[0046] When the answer to step 610 is “no”, the routine determines thatno port oxidation is required. Alternatively, when the answer to step610 is “yes”, the routine continues to step 612. Note that the exhaustmanifold flange temperature can be either measured from a sensor orestimated based on engine operating conditions. In one example, theflange temperature is estimated based on engine air-fuel ratio, coolanttemperature, and ignition timing.

[0047] Continuing with FIG. 6, in step 612, the routine determineswhether catalyst temperature (cat_tmp) is greater than a catalyst portoxidation threshold temperature (Ox_cat_tmp). Note that the catalysttemperature can be either measured from a catalyst temperature sensor,or estimated using various engine operating conditions such as, forexample: engine speed, engine air flow and ignition timing. When theanswer to step 612 is “no”, the routine determines that no portoxidation is required. Alternatively, when the answer to step 612 is“yes”, the routine continues to step 614.

[0048] In step 614, the routine monitors whether a throttle input hasoccurred. In one particular example, the throttle position is measuredand the routine determines whether the throttle position has bothincreased beyond a threshold value, and increased by predeterminedvalue. Alternatively, the routine could monitor whether the engine isstill in engine idle speed control. Further still, the routine couldmeasure various other parameters such as the pedal position, or thetransmission state to determine whether conditions have changed suchthat port oxidation is no longer required. Still other conditions can bewhether the engine airflow (or MAF) is greater than a predeterminedlimit value. Alternatively, cylinder charge can be the parameterutilized. Also, the routine could monitor whether the engine is still inengine idle speed control or detect a mass airflow from the mass airflowsensor 110 or other appropriate signals such as a intake manifold airpressure, an intake air temperature and an engine speed and compare themass airflow with a pre-selected threshold.

[0049] In yet another alternative embodiment, the pump is disabled at apre-selected estimated pump flow (which can be function of batteryvoltage, exhaust backpressure, and learned KAM values) which will causea delta lambda which is too small. Optionally, the pump can bere-enabled if the vehicle returns to a low load. Otherwise, the pump issimply turned on once per vehicle/engine startup. Alternatively, thepump could be left running (but deadheaded) until the catalyst is warmenough to allow for the return to idle condition.

[0050] When the answer to step 614 is “yes”, the routine discontinuesport oxidation. Alternatively, when the answer to step 614 is “no”, theroutine continues to step 616.

[0051] In step 616, the routine determines whether the exhaust manifoldflange temperature is greater than the lower port oxidation temperaturethreshold (Ox_low_tmp). When the answer to step 616 is “no”, the routinereturns to step 614. Alternatively, when the answer to step 616 is“yes”, the routine allows port oxidation and ends.

[0052] Referring now to FIG. 7, the port oxidation sequencing of step226 is now described. First, in step 710, the routine starts theoxidation timer. From step 710, the routine has two independent flowpaths starting from steps 712 and 730, respectively. In step 712, theroutine determines whether the oxidation timer (Ox_tm) is greater thanthe allowed fuel enrichment timer (fuel_on_tm). If the answer to step712 is “yes”, the routine continues to step 714 where the enrichmentfuel is enabled and the ignition timing is further retarded and theengine air flow is reduced. In this way, engine speed is maintained anda change in engine torque is minimized since the increased torque fromthe fuel enrichment is counteracted by the retarded ignition timing andreduced engine air flow. Air-fuel control during port oxidationenablement is further described in FIGS. 13-15, below.

[0053] Alternatively, when the answer to step 712 is “no”, the routinecontinues to step 716 where the routine determines whether a throttletransition has been detected. As described above herein, there arevarious methods to detect throttle transitions, such as based on a pedalposition, or various other methods. When the answer to step 716 is “no”,the routine returns to step 712. Otherwise, when the answer to step 717is “yes”, the routine continues to step 722 described below herein.

[0054] Continuing with FIG. 7, from step 714 the routine continues tostep 718. In step 718, the routine determines whether a throttletransition is detected similarly to step 716 (and step 614). If theanswer to step 718 is “yes”, the routine continues to step 722.Alternatively, when the answer to step 718 is “no”, the routinecontinues to step 720. In step 720, the routine determines whethereither the flange temperature is greater than the upper port oxidationthreshold (Ox_hi_tmp) or the high resolution oxidation timer(Oxtm_hires) is greater than a maximum allowed on time (MaxOntm). If theanswer to step 720 is “no”, the routine returns to step 718.Alternatively, when the answer to step 720 is “yes”, the routinecontinues to step 722 where the fuel enrichment and ignition timingretard are disabled.

[0055] Similarly, from step 710 the routine determines in step 730whether the oxidation timer is greater than an air pump ontime(airon_tm). When the answer to step 730 is “yes”, the routine continuesto step 732 and enables the air pump. Alternatively, when the answer tostep 730 is “no”, the routine continues to step 734 to detect whether athrottle transition has occurred. When the answer to step 734 is “no”,the routine continues to step 730. Alternatively, when the answer tostep 734 is “yes”, the routine continues to step 740 described belowherein.

[0056] From step 732, the routine continues to step 736 where adetermination is made as to whether a throttle transition has occurred.If the answer to step 736 is “yes”, the routine continues to step 740.Alternatively, when the answer to step 736 is “no”, the routinecontinues to step 738. In step 738, the routine determines whether theexhaust flange temperature (ext_fl) is greater than the upper portoxidation temperature threshold or whether the high resolution portoxidation timer is greater than the maximum on time in a manner similarto step 720. When the answer to step 738 is “no”, the routine returns tostep 736. Alternatively, when the answer to step 738 is “yes”, theroutine continues to step 742 to disable the air pump.

[0057] Referring now to FIG. 8, which is indicated in step 228 (FIG. 2),a routine for transitioning to the run mode is described. First, in step810, the routine ramps the enriched fuel towards stoichiometry viaclosed loop air-fuel ratio control using the exhaust gas oxygen sensors.Next, in step 812, the routine ramps the engine air and ignition timingwhile maintaining engine rpm via feedback idle speed control.

[0058] Referring now to FIG. 9, a graph of exhaust manifold temperatureindicates operation according to the present invention. In particular,FIG. 9 shows exhaust manifold temperature as a function of time. FIG. 9illustrates how the routine determines whether to enable the additionalair from the air pump with rich fuel combustion, known as portoxidation. In this example, the engine is started at T1. As the enginestarts, exhaust manifold temperature starts to rise and continues untiltime T2. At time T2, the exhaust manifold reaches the oxidationtemperature (approximately 500 to 800 degrees F.) at which timeconditions will support the exothermic reaction of air from the air pumpadded to the exhaust with rich combustion gases. At this point, exhaustmanifold temperature starts to rise at a higher rate until time T3. Atthis point, the temperature reaches the upper threshold and the air fromthe air pump is discontinued.

[0059] Referring now to FIG. 10, the graph illustrates measured exhaustair-fuel ratio upstream of the catalyst along with ignition timing(divided by a factor of 10). These values are plotted versus time afteran engine start. The value of one for the air-fuel ratio indicates nearstoichiometry. Further, a value of zero indicates base ignition timing.The graph shows how ignition timing is gradually retarded during thecold run mode. Then, at approximately six seconds, the pump is engagedand the port oxidation is enabled. Then, at approximately 15½ seconds,the pump is disengaged and the run mode starts, while ignition timing isadvanced. Note that while the measured exhaust air-fuel ratio indicatesrelatively close to stoichiometry during the port oxidation mode; thisis because the rich exhaust gases are reacting with the air from the airpump so that the sensor measures near stoichiometry.

[0060] As such, the present invention in one embodiment, operates lean,with ignition timing retarded, with no secondary air added via the airintroduction device if exhaust temperatures is less than a firsttemperature limit during an engine cold start. If temperature is greaterthan the first limit but less than a second limit, the engine operateswith secondary air and rich enough to support auto-ignition in theexhaust between the rich exhaust gas and the secondary air. Next, whentemperature is greater than the second limit, the engine can operate atstoichiometry without secondary air. In other words, after the enginehas reached the second limit, the engine can operate to maintain theexhaust gas about stoichiometry without additional air injection.

[0061] Finally, when operating with secondary air, the engine can bereturned to enleanment with retarded ignition timing if a non-idlecondition occurs. Alternative, engine air mass, cylinder charge, oranother indication of air amount can be used.

[0062] One reason for monitoring the idle/non-idle state, or some otherindication of whether airflow has increased beyond a threshold, is thatit prevents degradation in vehicle fuel economy. In other words, thepresent invention recognizes that to achieve the auto-ignition in theexhaust gas, the combustion air/fuel should preferably be richer thanapproximately 12:1. As such, if airflow increases substantially, thenthe amount of excess fuel increases proportionally. This can degradefuel economy if performed. Further, the present invention recognizesthat the air introduction device may not be able to provide enough airto go with all of this excess fuel. Thus, the disadvantage of incompleteburning of excess fuel is also avoided.

[0063] The present invention, in another embodiment, provides a methodfor operating an engine with an emission control device in an exhaustsystem of the engine, and an air introduction device coupled to theengine exhaust system. The method comprises operating the engine in afirst mode during cold idle conditions where the engine inducts a leanair-fuel mixture and ignition timing is retarded, operating the enginein a second mode, after said first mode, during cold idle conditionswhere the engine inducts a rich air-fuel mixture and the airintroduction device adds air to the engine exhaust, and exiting thesecond mode based on an increase in pedal position of a vehicle pedalactuated by a vehicle driver.

[0064] In this way, the less fuel efficient second engine operating modeis terminated when the engine operates at increased air flows. In otherwords, the rapid exhaust heating provided by a rich engine air-fuelratio and excess air added to the engine exhaust is carried out duringconditions where the impact on vehicle fuel economy is minimized. Assuch, the inventors herein have recognized that such rich operationshould be conducted during low engine air flow conditions to minimizethis negative impact on vehicle fuel economy.

[0065] This operation can be further explained with reference to FIG.11. The top graph of FIG. 11 shows engine air-fuel ratio as measured bya UEGO sensor upstream in the exhaust manifold. Note that the highinitial lean reading is because the sensor is not yet reached operatingtemperature. The second graph from the top illustrates exhausttemperature (exhaust manifold temperature in this example). The thirdgraph from the top illustrated the idle flag (on indicates the engine isin the idle mode). The bottom graph illustrates the air pump flag (onindicates the air pump is pumping air into the exhaust manifold).

[0066] Note that the top graph shows air-fuel ratio measured upstream ofthe air introduction device, whereas the embodiment described hereinuses a sensor that measures exhaust air-fuel ratio downstream of the airintroduction device. I.e., in the example described in these figures,the air-fuel ratio measured by a UEGO sensor located in place of sensor16 would show substantially a stoichiometric mixture from time1 on.Thus, the top graph of FIG. 11 could be considered a commanded incylinder air-fuel ratio, as described below.

[0067] The engine is first operated lean with retarded ignition timingafter the engine start until time1. At time1, the exhaust temperature(e.g., manifold temperature) reaches first threshold T1. At this time,since the engine is in the idle mode (note, in an alternative embodimentengine airflow is used in place of the idle flag), the enginetransitions to rich operation and starts adding air via the air pump inthe engine exhaust. In the first case (solid line), the engine exits theidle mode at time2. As such, the engine transitions back to leanoperation with retarded ignition timing.

[0068] The dashed line is a second case where the engine remains in theidle mode during the entire warm-up. At time3 in the second case, theengine reaches the second temperature threshold T2 indicating that theport oxidation mode is no longer required and thus transitions back tolean operation with retarded ignition timing. This continues untiltime5, where the temperature reached threshold T3, and the enginetransitions to oscillation about stoichiometry.

[0069] Returning to the first case, from time2 to time6, the engineoperates with retarded ignition timing. At time6, the temperaturereaches T3 and the engine then transitions to oscillations aboutstoichiometry.

[0070] Referring now to FIG. 12, an example air introduction device 209is shown. In this case, air pump 1212 is coupled to a control valve1210. The pump is either on or off based on pump command voltage Vc,which is fed to relay 1213, powered by the battery 1214. The airflow iscontrolled via the control valve 1210.

[0071] Note that this is just one configuration. In an alternativeconfiguration, the flow is simply controlled by adjustment of thevoltage applied to the pump.

[0072] Referring now to FIG. 13, a routine is described for controllingair-fuel ratio of the engine to compensate for air introduced via theair introduction device 209. This particular routine attempts tomaintain the mixture of combustion exhaust gases and air introduced viathe air introduction device at an overall mixture air-fuel ratio ofapproximately the stoichiometric value (relative air-fuel ratio of 1).Note, however, another air-fuel mixture target could be chosen,especially if UEGO air-fuel ratio sensors are used.

[0073] In general, the routine of FIG. 13 uses feedback from exhaustair-fuel ratio sensors to adaptively learn an estimate of air flowintroduced into the exhaust via the air introduction device. The routineonly enables learning of the pump air flow when the pump is on andfeedback air-fuel ratio control is executed. During this learning ofpump air flow, the control algorithms assume that errors due to fuelinjector offsets, errors from the mass air flow sensor, and bank to bankor cylinder to cylinder variations have already been learned andaccounted for during non-port oxidation mode. In this way, it ispossible to isolate the air flow error due to estimation errors of thepump air flow.

[0074] Referring now specifically to FIG. 13, in step 1310 the routineinitializes various variables stored in the KAM memory of controller 12.In particular, the variable (PETA_KAM) is initialized to the mostrecently updated value from previous engine operation. Upon initialengine production, this variable is set to zero.

[0075] In step 1312, the routine estimates air flow introduced via theair pump based on engine operating conditions. In other words, theroutine determines a feed-forward, or open-loop, estimate based onoperating conditions. The estimate of air pump flow (AIR_FLOW is basedon measured air mass flow (MAF) and battery voltage using the equationbelow.

AIR_FLOW_(—) CAL=FNAM(air mass)*FNVOLT(battery voltage)

[0076] where, FNAM is a calabratable function of air mass, or MAF, andFNVOLT is a calabratable function of battery voltage.

[0077] In an alternative embodiment, the air flow can be estimated basedon exhaust pressure. In other words, as described above herein, thepresent inventors recognize that the amount of air flow introduced viathe air pump (which is a non-positive displacement pump) is affected byexhaust back pressure and supply voltage. Further, various correctionscan be applied to account for variations in atmospheric pressure andatmospheric temperature.

[0078] Continuing with step 1313, the routine corrects the air pumpestimate based on learned data to obtain an adjusted air pump flowestimate (PETA as shown in the equation below.

PETA_INF_AM=AIR_FLOW_(—) CAL*(1+PETA_(—) KAM)

[0079] where AIR_FLOW_CAL is a calabratable adaptive gain parameter.

[0080] This adjusted estimate is based on the previous learned variable(PETA_KAM). Updating of this parameter is described below herein withparticular reference to step 1326. This approach thus uses a singlelearned value assuming that pump flow will approximately be reduced bythe same fraction under all conditions to provide the inferred air pumpflow used above. In particular, since the open loop estimate attempts tocompensate for variations in applied voltage, back pressure, andatmospheric conditions, only a single parameter is needed to compensatefor manufacturing variations and other slowly varying affects such aspump degradation.

[0081] However, in an alternative embodiment, the learned correctionvalue can be stored across various operating conditions such as, forexample: engine speed and load, or estimated exhaust gas temperature. Inother words, two values, one for each bank to account for bank-to-bankvariations in the air distribution system, could be used. Or, in theevent that separate pumps are used for each bank, multiple values couldbe used to account for differences in the pumps. Further, one couldassume a pre-determined bank-bank difference (when using one pump) andapply a bank specific multiplier while using the single value ofPETA_KAM for the whole system.

[0082] Continuing with FIG. 13, in step 1314 the routine calculates adesired end cylinder combustion air-fuel ratio (PETA_LAMBSE) as shown inthe equation below.

PETA _(—) LAMBSE=(AM*(PETA _(—) LAMBDA _(—) DES))/(AM+(PETA _(—) INF_(—) AM)),

[0083] where AM is the air mass determined from the mass air flow sensor(MAF), PETA_LAMBDA_DES is the desired mixture air-fuel ratio(PETA_LAMBDA), and PETA_INF_AM is the estimated air flow via the airintroduction device.

[0084] This desired end cylinder air-fuel ratio is thus calculated basedon the desired mixture air-fuel ratio (PETA_LAMBDA), measured air mass(AM) and the estimated air flow via the air introduction device(PETA_INF_AM). Note that according to the present invention, it ispossible to provide learning of air pump flow even if the mixture ratiois a value other than the stoichiometric ratio. As an alternative, aratio (PETA_RAT) can be calculated as shown in the equation below.

PETA _(—) RAT=AM/(AM+PETA _(—) INF _(—) AM)

[0085] In step 1316, the routine determines whether the EGO sensor 16has switched. If the answer to step 1316 is “no”, the routine continuesto monitor for an EGO switch. Alternatively, if the answer to step 1316is “yes”, the routine continues to step 1318. In step 1318, the routinesaves the final air-fuel ratio value determined from close loop air-fuelratio control when the EGO sensor switched (i.e., LAMBSE_SAVE is set tothe current LAMBSE, on a per bank basis if a multi bank engine is used).This value is calculated as described below herein with particularreference to FIG. 14. The value is saved, and further, the routine keepstrack of which bank ([BANK_TMP]) the air-fuel ratio value is saved fromin the case of a multi-cylinder bank engine. Then, in step 1320, theroutine calculates an error value (AIR_FUEL_ERROR) based on thedifference between the scheduled air-fuel ratio (PETA_LAMBSE) and therequired final air-fuel value necessary to maintain the measured mixtureratio (LAMBSE_SAVE) as shown by the equation below.

AIR_FUEL_ERROR=14.6*(LAMBSE_SAVE[BANK_(—) TMP]−PETA _(—) LAMBSE)

[0086] This difference seems to be a result of an error in the air-pumpflow estimation and the air flow air is calculated from the air-fuelratio error and the current fuel flow rate in step 1322 as shown in theequation below.

AIR_FLOW_ERROR=AIR_FUEL_ERROR*LBMF _(—) INJ[BANK TMP]*(NUMCYL/2)*N

[0087] where N is the engine speed, NUMCYL is the number of cylinders inthe engine, and LBMF_INJ is the Ibm mass of fuel injected from theinjectors.

[0088] Again, the routine accounts for a multi-bank system by selectingthe fuel flow from the bank corresponding to the saved air-fuel ratiovalue.

[0089] Then, in step 1324, the routine determines whether the pump flowis greater than a threshold value. Alternatively, the routine candetermine whether the ratio of airflow to air pump flow (PETA_RAT) isgreater than a threshold value. In particular, at low air pump flow(e.g., higher back pressure), other air-fuel ratio errors may overwhelmerrors due to changes in pump flow. Thus, according to the presentinvention, it is possible to only allow learning of pump flow when theseerrors will be minimized compared to the error in the pump flow. Thus,when the answer to step 1324 is “yes”, the routine continues to step1326 and updates the KAM value (PETA_KAM) as shown by the equationbelow.

PETA _(—) KAM=PETA _(—) KAM+(PETA _(—) KAM_GAIN)*(AIR_FLOW_ERROR)/(PETA_(—) INF _(—) AM)

[0090] where PETA_KAM_GAIN is a calabratable adaptive learning gain.

[0091] Step 1316 above assumes a switching sensor is used for sensor 16.An alternate would be to use a wide range sensor (UEGO) as sensor 16 andperform the learning at regular intervals instead of at sensor switchpoints. An advantage of the wide range sensor would be that updatescould be made more frequent, which would facilitate a more complexadaptive system where the learned correction could be evaluated as afunction of parameters such as exhaust backpressure or engine air mass,temperature, etc.

[0092] Referring now to FIG. 14A, a flowchart of a routine performed bycontroller 12 to generate fuel trim signal FT is now described. Adetermination is first made whether closed-loop air/fuel control is tobe commenced (step 1422) by monitoring engine operation conditions suchas temperature. When closed-loop control commences, signal EGO2S is read(step 1424) and subsequently processed in a proportional plus integralcontroller as described below.

[0093] Referring first to step 1426, signal EGO2S is multiplied by gainconstant GI and the resulting product added to products previouslyaccumulated (GI*EGO2S_(i-1)) in step 1428. Stated another way, signalEGO2S is integrated each sample period (i) in steps determined by gainconstant GI. During step 1432, signal EGO2S is also multiplied byproportional gain GP. The integral value from step 128 is added to theproportional value from step 1432 during addition step 1434 to generatefuel trim signal FT.

[0094] The routine executed by controller 12 to generate the desiredquantity of liquid fuel delivered to engine 10 and trimming this desiredfuel quantity by a feedback variable related both to sensor 80 and fueltrim signal FT is now described with reference to FIG. 14B. During step1458, an open-loop fuel quantity is first determined by dividingmeasurement of inducted mass airflow (MAF) by desired air/fuel ratio Afd(which is PETA_LAMBSE during port oxidation) which is typically thestoichiometric value for gasoline combustion. However, setting AFd to arich value will result in operating the engine in a rich state.Similarly, setting AFd to a lean value will result in operating theengine in a lean state. This open-loop fuel quantity is then adjusted,in this example divided, by feedback variable FV.

[0095] After determination that closed-loop control is desired (step1460) by monitoring engine operating conditions such as temperature(ECT), signal EGO1S is read during step 162. During step 1466, fuel trimsignal FT is transferred from the routine previously described withreference to FIG. 14A and added to signal EGO1S to generate trim signalTS.

[0096] During steps 1470-1478, a conventional proportional plus integralfeedback routine is executed with trimmed signal TS as the input. Trimsignal TS is first multiplied by integral gain value KI (step 1470), andthe resulting product added to the previously accumulated products (step1472). That is, trim signal TS is integrated in steps determined by gainconstant KI each sample period (i) during step 1472. A product ofproportional gain KP times trimmed signal TS (step 176) is then added tothe integration of KI*TS during step 178 to generate feedback variableFV. From this, the closed loop air-fuel value (LAMBSE) is thencalculated from the closed loop fuel injection amount (Fd) as:

LAMBSE=MAF/Fd.

[0097] Note that during port oxidation, second oxygen sensor 80 may nothave reached operating temperature. In this case, feedback generated inFIG. 14A is simply not used and step 1466 can be skipped.

[0098] In summary, the above adaptive air pump estimation routineprovides the following advantage. Whenever closed loop air-fuel controlis possible, the feedback correction can be used to adaptively learn thepump airflow. Further, this adaptively learned air-pump airflow can thenbe used in later engine starts, even if closed loop fuel control is notpossible, to maintain accurate air-fuel control even in the open loopsituation. Further, the controller 12 can adaptively learn errors fromthe fuel injectors, etc., during closed loop fuel control and non-portoxidation conditions. In this way, during learning in the port oxidationmode, the error can be attributed solely to errors in the air pump flowestimate.

[0099] Referring now to FIG. 15, example operation according to thepresent invention is described illustrating the adaptive learningaccording to FIGS. 13 and 14. In particular, the top graph of FIG. 15shows the feedback air-fuel ratio control input (LAMBSE) versus time asa solid line, and the stoichiometric air-fuel ratio as asoliddouble-dash line. The middle graph of FIG. 15 shows the feedbacksensor value, EGO versus time. Finally, the bottom graph of FIG. 15shows the pump flow estimate as a dashed line converging toward theactual pump flow (solid line).

[0100] At time t11, the engine enters closed loop feedback air-fuelcontrol based on the upstream air-fuel sensor 16, when the sensorswitches low, indicating the mixture air-fuel ratio is lean. Therefore,the control input (LAMBSE) is ramped more rich. Then, at time t12, thesignal EGO switches high, indicating the mixture of air and burnt gassesis rich. As such, the routine jumps the air-fuel ratio less rich, andbegins ramping toward lean (thus illustrating a P-1 control action).Further, at time t12 the routine updates the value of PETA_KAM thusadaptively learning a more accurate pump flow value. These processescontinue for each EGO switch, as illustrated at time t13 and t14. Asalso illustrated, the pump flow estimate oscillates around, andconverges toward the actual pump flow (which is not measured on thevehicle, but merely shown here to indicate that the estimate isconverging toward the actual value).

[0101] This concludes the detailed description. As noted above herein,there are various alterations that can be made to the present invention.

We claim:
 1. A method for controlling engine operation, the enginehaving an exhaust system including an air introduction device and anexhaust gas sensor, the method comprising: after an engine cold start,operating the engine with a rich air-fuel ratio and adding air via saidair introduction device; and during said operation: estimating an airamount introduced into the exhaust from said air introduction devicebased at least on an operating condition and an adaptively learnedparameter; determining if the sensor is operating; in response to saiddetermination, updating said adaptively learned parameter based on saidexhaust gas sensor; and injecting a fuel injection amount into theengine based on said estimated air amount.
 2. The method recited inclaim 1 wherein said sensor is an exhaust air-fuel ratio sensor.
 3. Themethod recited in claim 1 wherein said updating of said adaptivelylearned parameter occurs whenever the sensor switches from lean to richor from rich to lean.
 4. The method recited in claim 1 wherein saidoperating condition is at least one of a pump voltage or an ambientenvironmental condition.
 5. A method for controlling engine operation,the engine having an exhaust system including an air introduction deviceand an exhaust gas sensor, the method comprising: estimating an airamount introduced into the exhaust from said air introduction devicebased at least on an operating condition; injecting a fuel injectionamount into the engine based on said estimated air amount; measuring anexhaust gas condition based on the sensor; and adjusting said estimatingof said air amount based on said measured exhaust gas condition.
 6. Themethod recited in claim 5 wherein said exhaust gas condition is anexhaust gas air-fuel ratio.
 7. The method recited in claim 5 whereinsaid operating condition is ambient pressure.
 8. The method recited inclaim 5 wherein said operating condition is ambient temperature.
 9. Themethod recited in claim 5 wherein said operating condition pump voltage.10. The method recited in claim 5 wherein said operating condition isexhaust pressure.
 11. The method recited in claim 5 wherein saidoperating condition is engine airfl ow.
 12. The method recited in claim6 wherein said adjusting further comprises determining an air-fuel ratioerror between a desired exhaust mixture air-fuel ratio and said measuredexhaust gas air-fuel ratio, and determining a correction value based onsaid air-fuel ratio error.
 13. The method recited in claim 13 furthercomprises storing said correction value for future engine starts. 14.The method recited in claim 5 wherein said adjusting is enabled duringclosed loop air-fuel control conditions when the air introduction deviceis operating.
 15. The method recited in claim 14 wherein said airintroduction device is enabled when exhaust gas temperature is below afirst limit and above a second limit.
 16. The method recited in claim 5wherein said fuel injection amount is further based on a desired exhaustmixture air-fuel ratio, said mixture including burnt combustion gassesand introduced air.
 17. The method recited in claim 16 wherein saiddesired exhaust mixture air-fuel ratio is substantially stoichiometric.18. The method recited in claim 16 wherein said fuel injection amount isfurther based on said measured exhaust gas condition.
 19. A system,comprising: an engine having an exhaust system; an air pump coupled tosaid engine exhaust that pumps ambient air into said exhaust; an exhaustsensor coupled to said exhaust system that provides an indication ofexhaust air-fuel ratio; and a controller for estimating an air amountintroduced into said exhaust from said air introduction device based atleast on a pump voltage, injecting a fuel injection amount into theengine based on said estimated air amount, measuring an exhaust gascondition based on the sensor, and adjusting said estimating of said airamount based on said measured exhaust gas condition.
 20. The systemrecited in claim 19 wherein said exhaust gas sensor is an exhaust gasoxygen sensor.
 21. The system recited in claim 19 wherein said exhaustgas sensor is a switching exhaust gas oxygen sensor.
 22. The systemrecited in claim 19 further comprising a catalyst in the engine exhaust,said catalyst being downstream of said sensor and said air pump.
 23. Amethod for controlling engine operation, the engine having an exhaustsystem including an air introduction device and an exhaust gas sensor,the method comprising: estimating an air amount introduced into theexhaust from said air introduction device based at least on a pumpvoltage and atmospheric pressure; measuring an exhaust gas conditionbased on the sensor; injecting a fuel injection amount into the enginebased on said estimated air amount and said measured exhaust gascondition; determining a correction to said air amount estimate based onsaid measured exhaust gas condition and a desired exhaust gas mixtureair-fuel ratio, wherein said mixture includes burnt exhaust gas andintroduced air; and adjusting said fuel injection based on saidcorrection.
 24. A method for controlling engine operation, the enginehaving an exhaust system including an air introduction device and anexhaust gas sensor, the method comprising: estimating an air amountintroduced into the exhaust from said air introduction device based atleast on an operating condition; determining if the sensor is operating;in response to said determination, operating in a first mode when saidsensor is operating, said first mode comprising: injecting a first fuelinjection amount into the engine based on said estimated air amount;measuring an exhaust gas condition based on the sensor; and adjustingsaid estimating of said air amount based on said measured exhaust gascondition; and in response to said determination, operating in a secondmode when said sensor is not operating, said second mode comprising:injecting a second fuel injection amount into the engine based on saidadjusted estimated air amount.